The invention relates to the field of mobile radio systems. A subscriber in a mobile radio system, i.e. a subscriber station, for instance a mobile phone or a mobile station, may be registered in a radio network or system, whereby it communicates with the radio network via system channels comprised of control and traffic channels maintained by the base stations of the radio network.
In addition to the system channels in mobile radio systems, so-called direct mode channels can also be used in connection with a radio system, i.e. direct mode operation is applied. Subscriber stations using direct mode operation do not communicate directly with the radio network or its base stations. Direct mode channels are frequencies on which mobile phones or other means of communication are able to communicate directly with each other without the system, or via repeater stations either with the base stations of the system or with other mobile stations.
Direct mode channels are typically used in situations where, for instance, a group of portable mobile phones are communicating with each other at such a long distance from the base station that system channels cannot be used.
Another important way of using direct mode channels is to increase the capacity when the traffic in the system increases fast in some part of the service area of the system, for instance in some point-like part of the radio network.
A direct mode channel is referred to using the terms direct or simplex channel, or a simplex connection. A direct mode channel is a channel which is typically not at all used by the system. For example, it may be a channel with the same channel spacing as the channels of the system, for instance 12.5 kHz or 25 kHz. Among the mobile phones operating on a direct mode channel, the transmitting station has tuned its transmitter onto the channel and transmits speech or data information. The other mobile phones set to direct mode operation have tuned their receivers onto the same channel, whereby they are able to directly receive the transmission.
Operation on a direct mode channel may take place on the analog or digital modulation principle. A mobile phone transmitting on the channel may also transmit signalling information, such as information on rights of use and priorities or on the group operating on the channel. On the direct mode channel, an encryption may be carried out or plain speech can be transmitted.
Subscriber stations using direct mode operation communicate with other subscriber stations on a direct mode channel without being in direct contact with the base stations of the radio network. The subscriber stations using direct mode operation can also communicate with one another via repeater stations. A repeater station is a piece of equipment which re-transmits messages it has received. A repeater station may also forward traffic between the radio network and the subscriber stations.
According to the TETRA standard (TETRA=Trans-European Trunked Radio) known in the field of mobile radio systems, frequency accuracy of a base station (BS) should be within +/- 0.2 ppm. Therefore, on a frequency of 800 MHz, for example, the permitted frequency tolerance is +/- 160 Hz. Frequency accuracy indicates how accurate the actual frequency of the transmitter is in relation to the nominal frequency determined for it. Mobile stations (MS) should have such a frequency accuracy that the basic accuracy is +/- 2 ppm, which is adjusted to the signal received from the base station, which has the accuracy of +/- 0.2 ppm. Thus, an accuracy of +/- 160 Hz is required for the mobile station to be accurate. This means that the mobile station MS is not allowed to transmit unless its transmit frequency falls within the limits set. In direct mode operation, all the mobile stations may not be able to receive the base station, and therefore cannot synchronize with the frequency on which it is transmitting. Synchronizing with the frequency denotes that the transmit frequency is tuned to within +/- 0.2 ppm in comparison with the receive frequency. The transmit and receive frequencies may be on different frequency channels, but at a fixed divisor from each other. In direct mode operation, however, operation takes place on the same frequency channel. The synchronization with a frequency is carried out by calculating the phase error by means of software on the basis of the received signal and by low-pass filtering the phase error obtained, whereby frequency error will be obtained. Minimizing the frequency error corresponds to synchronization with the frequency.
Previously, the idea behind frequency synchronization in direct mode traffic was that the transmitter of the receiving, i.e. the second mobile station MS2, always synchronizes with the frequency of the transmitting, i.e. the first mobile station MS1, or that a correction is not carried out at all. If the receiving mobile station MS2 of the transmission operates as the transmitting party the next time, the frequency it is using must act as the synchronizing frequency for the first mobile station MS1 which is at that moment the receiving party. The mobile station MS is not, however, capable of synchronizing exactly with the receive frequency, which means that the frequency information it transmits differs from the frequency information it received earlier. As a result of a number of changes in signal source, the frequency may drift progressively further away from the nominal frequency and possibly off the allocated frequency channel altogether.